Batteries have been utilized for many years as electrical power generators in remote locations. Through the controlled movement of ions between electrodes (anode and cathode), a power circuit is generated, thereby providing a source of electricity that can be utilized until the excess ions in one electrode are depleted and no further electrical generation is possible. In more recent years, rechargeable batteries have been created to allow for longer lifetimes for such remote power sources, albeit through the need for connecting such batteries to other electrical sources for a certain period of time. All in all, however, the capability of reusing such a battery has led to greater potentials for use, particularly through cell phone and laptop computer usage and, even more so, to the possibility of automobiles that solely require electricity to function.
Such batteries typically include at least five distinct components. A case (or container) houses everything in a secure and reliable manner to prevent leakage to the outside as well as environmental exposure inside. Within the case are an anode and a cathode, separated effectively by a separator, as well as an electrolyte solution (low viscosity liquid) that transport ions through the separator between the anode and cathode. The rechargeable batteries of today and, presumably tomorrow, will run the gamut of rather small and portable devices, but with a great deal of electrical generation potential in order to remain effective for long periods between charging episodes, to very large types present within automobiles, as an example, that include large electrodes (at least in surface area) that must not contact one another and a large number of ions that must consistently and constantly pass through a membrane to complete the necessary circuit, all at a level of power generation conducive to providing sufficient electricity to run an automobile motor. As such, the capability and versatility of battery separators in the future must meet certain requirements that have yet to be provided within the current industry.
Generally speaking, battery separators have been utilized since the advent of closed-cell batteries to provide necessary protection from unwanted contact between electrodes as well as to permit effective transport of electrolytes within power generating cells. Typically, such materials have been of film structure, sufficiently thin to reduce the weight and volume of a battery device while imparting the necessary properties noted above at the same time. Such separators must exhibit other characteristics, as well, to allow for proper battery function. These include chemical stability, suitable porosity of ionic species, effective pore size for electrolyte transfer, proper permeability, effective mechanical strength, and the capability of retaining dimensional and functional stability when exposed to high temperatures (as well as the potential for shutdown if the temperature rises to an abnormally high level).
In greater detail, then, the separator material must be of sufficient strength and constitution to withstand a number of different scenarios. Initially, the separator must not suffer tears or punctures during the stresses of battery assembly. In this manner, the overall mechanical strength of the separator is extremely important, particularly as high tensile strength material in both the machine and cross (i.e., transverse) directions allows the manufacturer to handle such a separator more easily and without stringent guidelines lest the separator suffer structural failure or loss during such a critical procedure. Additionally, from a chemical perspective, the separator must withstand the oxidative and reductive environment within the battery itself, particularly when fully charged. Any failure during use, specifically in terms of structural integrity permitting abnormally high amounts of current to pass or for the electrodes to touch, would destroy the power generation capability and render the battery totally ineffective. Thus, even above the ability to weather chemical exposure, such a separator must also not lose dimensional stability (i.e., warp or melt) or mechanical strength during storage, manufacture, and use, either, for the same reasons noted above.
Simultaneously, however, the separator must be of proper thickness to, in essence, facilitate the high energy and power densities of the battery, itself. A uniform thickness is quite important, too, in order to allow for a long life cycle as any uneven wear on the separator will be the weak link in terms of proper electrolyte passage, as well as electrode contact prevention.
Additionally, such a separator must exhibit proper porosity and pore sizes to accord, again, the proper transport of ions through such a membrane (as well as proper capacity to retain a certain amount of liquid electrolyte to facilitate such ion transfer during use). The pores themselves should be sufficiently small to prevent electrode components from entering and/or passing through the membrane, while also allowing, again, as noted above, for the proper rate of transfer of electrolyte ions. As well, uniformity in pore sizes, as well as pore size distribution, provides a more uniform result in power generation over time as well as more reliable long-term stability for the overall battery as, as discussed previously, uniform wear on the battery separator, at least as best controlled in such a system, allows for longer life-cycles. It additionally can be advantageous to ensure the pores therein may properly close upon exposure to abnormally high temperatures to prevent excessive and undesirable ion transfer upon such a battery failure (i.e., to prevent fires and other like hazards).
As well, the pore sizes and distributions may increase or decrease the air resistance of the separator, thus allowing for simple measurements of the separator that indicate the ability of the separator to allow adequate passage of the electrolyte present within the battery itself. For instance, mean flow pore size can be measured according to ASTM E-1294, and this measurement can be used to help determine the barrier properties of the separator. Thus, with low pore size, the rigidity of the pores themselves (i.e., the ability of the pores to remain a certain size during use over time and upon exposure to a set pressure) allows for effective control of electrode separation as well. More importantly, perhaps, is the capability of such pore size levels to limit dendrite formation in order to reduce the chances of crystal formation on an anode (such a lithium crystals on a graphite anode) that would deleteriously impact the power generation capability of the battery over time.
Furthermore, the separator must not impair the ability of the electrolyte to completely fill the entire cell during manufacture, storage and use. Thus, the separator must exhibit proper wicking and/or wettability during such phases in order to ensure the electrolyte in fact may properly transfer ions through the membrane; if the separator were not conducive to such a situation, then the electrolyte would not properly reside on and in the separator pores and the necessary ion transmission would not readily occur. Additionally, it is understood that such proper wettability of the separator is generally required in order to ensure liquid electrolyte dispersion on the separator surface and within the cell itself. Non-uniformity of electrolyte dispersion may result in dendritic formations within the cell and on the separator surface, thereby creating an elevated potential for battery failures and short circuiting therein.
There is also great concern with the dimensional stability of such a separator when utilized within a typical lithium ion cell, as alluded to above. The separator necessarily provides a porous barrier for ion diffusion over the life of the battery, certainly. However, in certain situations, elevated temperatures, either from external sources or within the cell itself, may expose susceptible separator materials to undesirable shrinking, warping, or melting, any of which may deleteriously affect the capability of the battery over time. As such, since reduction of temperature levels and/or removal of such battery types from elevated temperatures during actual utilization are very difficult to achieve, the separator itself should include materials that can withstand such high temperatures without exhibiting any appreciable effects upon exposure. Alternatively, the utilization of combinations of materials wherein one type of fiber, for instance, may provide such a beneficial result while still permitting the separator to perform at its optimum level, would be highly attractive.
To date, however, as noted above, the standards in place today do not comport to such critical considerations. The general aim of an effective battery separator is to provide such beneficial characteristics all within a single thin sheet of material. The capability to provide low air resistance, very low pore size and suitable pore size distribution, dimensional stability under chemical and elevated temperature environments, proper wettability, optimal thickness to permit maximum battery component presence in the smallest enclosure possible, and effective overall tensile strength (and preferably isotropic in nature), are all necessary in order to accord a material that drastically reduces any potential for electrode contact, but with the capability of controlled electrolyte transport from one portion of the battery cell to the other (i.e., closing the circuit to generate the needed electrical power), in other words for maximum battery output over the longest period of time with the least amount of cell volume. Currently, such properties are not effectively provided in tandem to such a degree. For instance, Celgard has disclosed and marketed an expanded film battery separator with very low pore size, which is very good in that respect, as noted above; however, the corresponding air resistance for such a material is extremely high, thus limiting the overall effectiveness of such a separator. To the contrary, duPont commercializes a nanofiber nonwoven membrane separator that provides very low air resistance, but with overly large pore sizes therein. Additionally, the overall mechanical strengths exhibiting by these two materials are very limiting; the Celgard separator has excellent strength in the machine direction, but nearly zero strength in the cross (transverse) direction. Such low cross direction strength requires very delicate handling during manufacture, at least, as alluded to above. The duPont materials fare a little better, except that the strengths are rather low in both directions, albeit with a cross direction that is higher than the Celgard material. In actuality, the duPont product is closer to an isotropic material (nearly the same strengths in both machine and cross directions), thus providing a more reliable material in terms of handling than the Celgard type. However, the measured tensile strengths of the duPont separator are quite low in effect, thus relegating the user to carefully maneuvering and placing such materials during manufacture as well. Likewise, the dimensional stability of such prior battery separators are highly suspect due to these tensile strength issues, potentially leading to materials that undesirably lose their structural integrity over time when present within a rechargeable battery cell.
Thus, there still exists a need to provide a battery separator that simultaneously provides all of these characteristics for long-term, reliable, lithium battery results. As such, although such a separator exhibiting low air resistance and low pore size, as well as high tensile strength overall and at relatively isotropic levels, proper chemical stability, structural integrity, and dimensional stability (particularly upon exposure to elevated temperatures), although highly desired, to date there has been a decided lack of provision of such a prized separator material. Additionally, a manner of producing battery separators that allows for achieving such desired targeted property levels through efficient manufacturing processes would also be highly desired, particularly if minor modifications in materials selection, etc., garners such beneficial results and requirements on demand; currently, such a manufacturing method to such an extent has yet to be explored throughout the battery separator industry. As such, an effective and rather simple and straightforward battery separator manufacturing method in terms of providing any number of membranes exhibiting such versatile end results (i.e., targeted porosity and air resistance levels through processing modifications on demand) as well as necessary levels of mechanical properties, heat resistance, permeability, dimensional stability, shutdown properties, and meltdown properties, is prized within the rechargeable battery separator industry; to date, such a material has been unavailable.